The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including oxides of nitrogen and greenhouse gases such as carbon dioxide. Accordingly, need exists for significant improvement in the efficiency of fuel utilization for automotive powertrains to reduce greenhouse gas emissions while achieving low levels of NO.sub.x.
Current automobiles and light trucks primarily utilize gasoline-fueled engines which operate on the Otto cycle. These engines are characterized by operation near stoichiometry, i.e., operation where the amount of fuel and air that is metered to the engine is controlled such that just enough oxygen is available to fully burn the available fuel. Stoichiometric operation is maintained by reading the output of an oxygen sensor in the exhaust and adjusting the fuel accordingly. This "closed loop" operation is combined with an exhaust catalyst that is able to both reduce the NO.sub.x and oxidize unburned fuel emissions. Very low levels of these emissions are thus obtained.
Larger trucks and buses primarily utilize Diesel-fueled engines which operate on the Diesel cycle. These engines are characterized by unthrottled air induction, i.e., lean or excess-air combustion, high compression, and direct fuel injection. Major effort is currently being focused on adapting such engines to automobiles and light trucks because of their superior fuel economy characteristics. However, because these engines operate with excess air, conventional reduction catalysts cannot be used and exhaust NO.sub.x emissions remain high, especially at higher load operation.